• Preheat the oven to 200°C/400°F.

For the filling

• Melt the butter in a large pan over medium heat. Add the leeks with a pinch of salt and fry for about 6-8 minutes, until softened but not colored.
• Add the chicken, mustard, and white wine. Cook until the alcohol has burned off.
• Add the chicken stock and bring to a boil. Simmer for about 5 minutes until it has thickened slightly, then stir in the sour cream and season to taste.

For the dumpling biscuits

• Place the flour, baking powder, and a good pinch of salt in a mixing bowl. Add the butter and use your fingertips to rub it in, until the mix resembles fine breadcrumbs.
• Add three quarters of the cheese and rub that in roughly.
• Add the milk and bring together into a soft dough. Roll on on a floured surface and cut into triangles.

To assemble and bake

• Place the filling in a deep, ovenproof dish. Float the dumpling biscuits on top and scatter with the reserved cheese.
• Bake for about 15 minutes until the dumpling biscuits have risen and are golden on top, and the filling is bubbling around the edges.